There is a general desire to control the emission of particulates in industrial gas streams in light of government regulations designed to curtail pollution. In the area of oil refinery operations, one particular area of concern regarding particulate emissions lies in the flue gas exiting catalyst regenerators of fluid catalytic cracking (FCC) units.
The FCC process, now more than 50 years old, has undergone continuous improvement and remains the predominant methodology of gasoline production in many refineries. This gasoline, as well as lighter products, is formed as the result of cracking heavier (i.e. higher molecular weight), less valuable hydrocarbon feed stocks such as gas oil. A general outline of the FCC process is provided below, although it is to be appreciated that the FCC process is a large and complex process involving many factors that are not addressed.
In its most general form, the FCC process involves use of a reactor that is closely coupled with a catalyst regenerator, followed by downstream hydrocarbon product separation. The catalyst regenerator collects spent catalyst having coke on the surface thereof, combusts the coke from the surface of the spent catalyst to refresh the catalyst, and returns the refreshed catalyst to the reactor. One specific feature of the FCC process is the continuous fluidization and circulation of large amounts of catalyst having an average particle diameter of about 50-100 microns, equivalent in size and appearance to very fine sand. For every ton of cracked product made, approximately 5 tons of catalyst are needed, thus highlighting the significance of the catalyst regenerator in the FCC process. Coupled with the need for a large inventory and recycle of a small particle diameter catalyst is the ongoing challenge to prevent this catalyst from exiting the reactor and/or catalyst regenerator system into effluent streams that are released into the environment.
Overall, the use of cyclone separators internal to both the reactor and catalyst regenerator has been developed to provide over 99% separation efficiency of solid catalyst. Regenerators generally include first and second (or primary and secondary) stage separators for the purpose of preventing catalyst contamination of regenerator flue gas, which is essentially the resulting combustion product of catalyst coke in air. While normal-sized catalyst particles are effectively removed in the first and second stage separators, fines material (generally catalyst fragments smaller than about 50 microns resulting from attrition and erosion in the harsh, abrasive reactor and catalyst regenerator environments) is substantially more difficult to separate. As a result, the regenerator flue gas may contain unacceptably high particulate concentrations, which may not only pose potential environmental concerns but which may also damage devices downstream of the regenerator flue gas.
A further reduction in regenerator flue gas fines loading is therefore often warranted, and may be obtained from a third stage separator (TSS) device containing a manifold of cyclones. Electrostatic precipitators are known to be effective for this gas/solid separation but are far more costly than a TSS, which relies on the induction of centripetal acceleration to a particle-contaminated gas stream, forcing the higher-density particles to the outer edges of a spinning vortex. To be efficient, a third stage separator device for a regenerator flue gas stream will normally contain many individual cyclone separators installed within a single vessel acting as a manifold.
In the area of cyclone design, significant emphasis has been placed on so-called “reverse flow” types where incoming gas is added around a gas outlet tube extending from the inlet side of a cylindrical cyclone body. Particle-rich gas can be withdrawn from discharge openings in the sidewall of the cyclone body, while clean gas essentially reverses flow from its initial path toward the end of the cyclone body opposite the gas inlet, back toward the gas outlet. The gas outlet is a tube normally concentric with, and located within the cyclone body. Unfortunately, the requirement by itself for a gas stream to reverse direction and exit the cyclone body on the same side as the gas inlet imposes flow disturbances that are not easily overcome.
“Uniflow” cyclone separators are known that eliminate the re-entrainment of solids associated with the reversal of gas direction. In this case, the cyclone separator is disposed between a first tube sheet and a second tube sheet, with the space between the first tube sheet and second tube sheet designed to collect separated particles. In the uniflow cyclone separators, clean gas moves continually downward and exits the cyclone body through a cyclone gas outlet that extends below the second tube sheet, which serves as the physical boundary between clean gas and the separated particles in the space between the first tube sheet and the second tube sheet. Separated particles are removed from discharge openings in the cyclone body into the space between the first tube sheet and the second tube sheet.
Variations in the design of uniflow cyclone separators have involved modification to the cyclone gas outlet as well as modification to the discharge opening configuration to promote uniform flow patterns within the cyclone body. For example, beveled edges of the discharge openings have been proposed to align the discharge openings with a flow path of entrained particles within the particle-contaminated gas stream. However, discharge openings of existing cyclone separators are still subject to design flaws due to their shape. In particular, existing discharge openings are generally rectangular in shape. Entrained particles uniformly impact a lagging edge of the discharge openings in relation to a flow path of entrained particles within the particle-contaminated gas stream, often resulting in erosion of the lagging edge and leading to widening of the discharge openings. Widening of the discharge openings is undesirable due to the effect of such widening on flow patterns within the cyclone body.
Accordingly, it is desirable to provide cyclone separators having a discharge opening that is redesigned to minimize erosion and address issues with widening of the discharge openings due to erosion. In addition, it is desirable to provide separator devices including the cyclone separators having the redesigned discharge opening. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.